Endpoint for getting valid checks for the frontend

[dcl10]

Description

Create an endpoint that returns the valid pendera checks. The checks returned should be different depending on the type.

Tasks

• [x] create a GET endpoint that returns the valid checks
• [x] use a parameter for the dtype, which will determine what to send back.
  • [x] numeric should allow <, >, <=, >= and ==
• [x] return a list of checks

[kuraisle]

Closed with #27